The proposed research aims to determine the antigenic strength, frequency of occurrence, and interrelationships of virus-induced tumor-specific antigens in MTV-free and MTV-infected nodules and tumors. The low frequency of expression of virus-induced tumor specific antigens in tumors arising from virus-free nodules transplanted into MTV-positive C3H/Ki mice suggest that the interrelationships between the phenotype of the target cell and virus infection might influence antigenic expression. Alternatively, the C3H/Ki subline might behave differently in the frequency of expression of tumor specific antigens than the C3H/He subline, where the initial experiments were performed. These two parameters are being investigated extensively.